MOSFET integrated circuits require their semiconductor substrate to have a fixed voltage bias. Early IGFET integrated circuits employed the use of an external voltage bias source requiring an additional electrical connection to the substrate. The prior art improved upon this approach through the development of the field effect charge pump for injecting charge into the substrate, such as is disclosed in U.S. Pat. No. 3,794,862, a cross-sectional view of which is shown in FIG. 1. The prior art charge pump comprised a P type substrate 2 in which has been formed the N type source region 4, produced by known processes and techniques. For example, phosphorous impurities can be diffused through openings in a thick silicon dioxide dielectric layer disposed on the surface of a single crystal silicon substrate to form the N+ source regions in the P type substrate. A thin oxide layer 22 is then grown in the gate region overlapping the source 4. The gate electrode 14 is deposited over thin oxide layer 22. The source electrode 4, supplies negative charge to be injected into the substrate 2, when the potential of the gate electrode exceeds the thin oxide threshold voltage, V.sub.T, thereby inverting the conductivity of the substrate beneath the thin oxide layer 22. The supplying of negative charge for the substrate may be described as "pumping" charge into the substrate. The clock signal .phi..sub.1, connected to the gate electrode 14, controls the charge pumping operation. This biasing circuit is employed to inject charges into a substrate-capacitor series combination with one side of the capacitor being grounded.
A square wave voltage signal .phi..sub.1 is used to pump minority carriers from the diffusion 4 into the substrate 2. When this prior art charge pumping circuit is used to bias the substrate of a semiconductor chip containing a large number of reverse biased diffusions, its operation deteriorates since a fraction of the pump current is lost to the leakage current of these diffusions. Since these reverse leakage currents are erratic and difficult to reduce, it is then of great interest to insure that the charge pump is able to drive current well above the leakage level. One way to do this might be to increase the area of the gate region 22. However, this is expensive in terms of silicon area. A better way would be to improve the charge pump efficiency that is, reducing the fraction of electrons that are lost back to the source diffusion 4 during each pumping cycle, thereby increasing the net current output.
The principles governing charge pumping in MOS transistor devices are explained in J. S. Brugler, et al., "Charge Pumping in MOS Devices," IEEE Transactions on Electron Devices, Vol. ED-16, No. 3, pages 297-303, 1969. There, the charge pump current is expressed by the sum of the following components: EQU I.sub.p = A.f.sub.p .multidot.(C.sub.ox (V.sub.G -V.sub.T) + qN.sub.ST) (1)
here, A is the area of the plate 14 gating the source diffusion, f.sub.p is the frequency of the pumping wave-form .phi..sub.1, C.sub.ox is the thin oxide capacitance per unit area, V.sub.G is the most positive voltage of the signal .phi..sub.1, V.sub.T is the threshold voltage corresponding to the thin oxide, q is the unit charge of an electron, and N.sub.ST is the electron surface state density. The first term within the parenthesis of equation 1 is the component due to the compensation of the injected minority carriers by majority carriers from the bulk of the substrate 2 and the second component in equation 1 is due to recombination through fast surface states. If a charge pump with high current capability is desired, it is necessary to let the first component of equation 1 become dominant. This first term of equation 1 is called the geometric component since conditions favorable to its existance depend on the geometry of the diffusion 4. It is shown in the Brugler, et al., reference that the best geometry is an annular gated diode with its junction at its center. This is because the reflow of charge from the substrate into the junction meets an increasingly larger resistance with diminishing radius due to the reduced cross-sectional area that the current has to flow through. Therefore, charge losses to the source diffusion 4 can be minimized and the pump efficiency increased. However, this approach has a significant drawback in that circular shapes are difficult to render in the photolithographic masking processes necessary to construct the diffused region 4. What the semiconductor art needs is an improved charge pumping structure which will enhance the efficiency of the field effect transistor type charge pump.